Throw a free concert and everyone will show up. Girls with glowing Hula-Hoops. Stroller-pushing parents. Grizzled men in faded ZZ Top T-shirts. That guy in the skintight tie-dye bodysuit.

A crowd of 50,000 squeezed through the gates of Merriweather Post Pavilion on Saturday for Virgin Mobile FreeFest, a gathering of wildly disparate tribes. They came to hear 22 acts, including Southern rock newcomers Alabama Shakes, rap great Nas, pop alchemist Santigold, the eternally hirsute ZZ Top, and dueling headlining sets from former White Stripes frontman Jack White and dubstep prince Skrillex.

The sounds were spread across three stages — the venue’s usual Pavilion stage, a second stage built in a neighboring field to the west and a third, smaller stage for DJs dubbed “the Dance Forest.”

And for the fourth year running, organizers gave tickets away online for free.

Fans swarmed the place to let it all hang out. Or maybe just keep it tucked in. There were furry boots and flip-flops. Gold laméthongs and cargo shorts. Fishnet stockings and mesh lacrosse jerseys. Orioles caps outnumbered Nationals caps, outnumbered dragon-green caps with purple fins, of which there was at least one. For 11 colorful hours, this year’s FreeFest was a place to see and be seen — and hopefully find some music that matched your outfit.

12:01 p.m.

“Good morning,” says Will Eastman, greeting the early birds assembled on a dance floor of dry oak leaves and dirt. Eastman’s band, Washington-based Volta Bureau, is the first act of the festival, and its thumping DJ set in the Dance Forest foreshadows a day of unrelenting rhythm. Wearing a zebra-print blouse and a constellation of stars tattooed up the back of her neck, Liz Carr, 27, laughs when a falling acorn beans her in the head. She doesn’t stop dancing, though, insisting that she’s pacing herself. “Our goal,” she says, “is to not be blacked out by the time Skrillex comes on.”

2:32 p.m.

Virgin founder Sir Richard Branson has just returned backstage after a quick mosey across the grounds, darting through already thickening crowds. “Just saying hello to some of the punters,” Branson says, smiling. “They’re very grateful, as if I’ve done all the hard work by myself.” He touts FreeFest’s charity work with homeless youth and says that he’s excited to get back out and see the bands.

Fans are already stomping circles on the lawn to the neo-bluegrass of Trampled By Turtles. Down in the Pavilion seats, fans bat at a beige, malformed balloon, trying to keep it afloat. The guy who figures out it’s an inflated condom lets it fall to the ground.

3:38 p.m.

For some, FreeFest is a place to test-drive your Halloween costume. For the Dismemberment Plan, it’s a place to test-drive new tunes — all while trying to win fans as they hover hundreds of feet over the proceedings.

Travis Morrison, frontman of the recently reunited Washington post-punk troupe, persuades the crowd in front of the stage to salute those riding the neighboring Ferris wheel by shouting, “HI, FERRIS WHEEL!”

A girl in the lime-green carriage waves back toward the stage, then leans over the rail to talk to her pals in the purple carriage below. Morrison shouts in mock-terror: “Don’t lean over. . . . I’m afraid of heights!”

4:56 p.m.

It’s the festival’s only costume change. Santi White, the Philadelphia-born singer who mixes ska, new wave and dancehall reggae as Santigold, switched from a limesicle dress to a black-and-white one. Meanwhile, her backup dancers are trying to lasso a horse, or rather, two people wearing a two-person horse costume. Her dance routines provide the first moments of the day where the playfulness in the audience is matched by some playfulness onstage, and it goes over well — save for a quartet of buddies dressed as the Teenage Mutant Ninja Turtles who wander off to see something else, apparently nonplussed.

6:55 p.m.

Erik Berry plays mandolin for Trampled By Turtles, but now he’s another fan in the crowd. “Our management convinced us to play by saying ZZ Top would be here,” Berry says. But how did ZZ Top get on the bill? The band was booked to open a Jack White gig at Merriweather. When promoters asked White to change plans and headline FreeFest, White requested that ZZ Top play, too.

The band eventually takes the stage and tears into its diesel-doused repertoire, including a new cover of “25 Lighters,” the Houston rap anthem by DJ DMD, Lil Keke and Fat Pat. As the rock band covers the rap song, the bluegrass musician bobs his head.

7:34 p.m.

It’s hard to see when you’re dancing in the dark, so when the sun sets, the fight against anonymity begins. In the Dance Forest, jugglers in capes start tossing glowing orbs. One man has glow sticks tucked into the folds of his turban. But Connor Kowalewski, 18, and his friends shine the brightest. They’re wrapped in strands of white Christmas lights. “Three double-A batteries,” Kowalewski says, patting the power source in his pocket.

As DJs Porter Robinson and Zedd unleash a beefed-up version of the viral hit “Gangnam Style,” Kowalewski and his radiant pals bounce off toward the stage, parting the crowd with their glow.

8:59 p.m.

After a day spent ogling each other, all eyeballs are now locked on Jack White. It’s his first area solo appearance since the White Stripes went kaput, and his six backing musicians, all blank-faced women dressed in ghostly white frocks, make his old band’s songs sound bigger, nastier, bruised-up and brash. White mauls his guitars for 50 riveting minutes, but is immediately called back for an encore by thousands of voices WHOA-OH-OH-ing the guitar riff of “Seven Nation Army.” Baltimore Ravens fans chant this melody during games at M&T Bank Stadium.

Were they always just practicing for this?

10:57 p.m.

Plastic cups crunching underfoot, Jamie Bryan, 22, is headed to the parking lot. “Incredible,” she says of the Skrillex performance that just ended, a 90-minute bombardment of riotous dubstep songs with sci-fi lasers, flames, smoke and sparks to match.

But now it’s quiet. The only things that still qualify as loud are the declarations adorning fans’ T-shirts: “CAUTION! WATCH YOUR DUBSTEP,” “SEX DRUGS AND DUBSTEP,” “KEEP CALM AND DUBSTEP ON.”

But the six letters printed on Bryan’s tank top capture the exodus from Merriweather best: “THE END.”

Eleven hours of crazy costumes and a variety of musicians and their fans, all for free. C9